Don't know if you read my other post here, but I used the Canon 35 f2 (non-IS) on a cruise recently; in fact roughly 75% of my 550 shots were taken with it. It was my go to lens inside the ship as I didn't want to use flash.

Of course not all shots turned out as I hoped, surprise surprise, but one of the big reasons was the shutter speed resulting in blurred pics of people, not obvious but there never the less. I can remember wishing I had the f1.4 of my Siggy 50 so I could double my shutter speed.

When IS first came out I was a big fan, now I turn it off unless I "really" need it. But IS just won't give you faster shutter speeds to halt subject motion no matter how good it, 4 stops even.

The thing is, a more shallow DOF comes along with that extra stop. For me, that's a good thing (in fact, why I choose the Sigma over the Canon). For others, not so much.

You can use the same shutter speed on both, but it will simply result in more noise at f/2 than f/1.4. However, the Canon is sharper at f/2 than the Sigma at f/1.4, so it can withstand a bit more NR (noise reduction) to make up at least some of the difference.

On the other hand, one of the reasons I really wanted IS was to take handheld pics of static subjects with motion in the scene, like this:

Canon 5D + 24 / 1.4L @ f/5.6, 1/8, ISO 100

it's just that I think I'd be using the more shallow DOF of f/1.4 more often than taking shots like that. It's unfortunate, indeed, that Canon does not offer sensor IS, so I wouldn't have had to choose.